Many different kinds of shaped articles such as films, filaments, sheets, containers and tubing may be made from various polymers and polymeric compositions. In many cases, the shaped articles are required to have special properties and it has often been necessary to develop special blends of various resins in order to meet particular requirements. For example, it may be desired to make a film which has good strength, tear resistance and toughness while at the same time possessing good vapor barrier properties. If there is no single polymer which will confer this desired combination of properties, it may be necessary to use a blend of polymers or to fabricate a laminar film in which the individual layers of the laminate combine to provide the desired combination of properties. Another approach which has recently received attention is to employ a heterogeneous blend of polymers in which one polymer is dispersed in another polymer in the form of thin, parallel and overlapping layers embedded in the continuous phase. The dispersed polymer may be used to provide vapor barrier properties, as disclosed in U.S. Pat. No. 4,410,482 or other properties, such as strength, stiffness, or tear resistance. Because a dispersed phase which generally will have a different refractive index to the continuous phase is present, the optical properties of the blend will generally be affected although if transparency is to be retained, suitable choice of blend components may minimize the optical losses which occur at the phase boundaries. Generally, the heterogeneous blends will be ternary compositions comprising a base polymer which constitutes the continuous phase, a dispersed polymer and a compatibilizer which is used to balance the phase boundary. Ternary blends of this kind are described, for example, in U.S. Pat. Nos. 3,857,754 and 4,410,482. In the blends disclosed in U.S. Pat. No. 3,857,754, the compatibilizing polymer is one which contains a carbonyl group in its main or side chain, for example, polyacrylic acid, polymethacrylate esters, polyamides and various polyesters. The compatibilizers used in the compositions of U.S. Pat. No. 4,410,482 are generally ionomers derived from acidic copolymers.
Although polymer blends of this kind are capable of providing varied combinations of properties in the finished article, one problem has been that the dispersed phase will not always distribute itself in the continuous phase in a manner which provides the optimium combination of properties. When the resin blend is passed through the extruder, the extrudate will have a stratified structure consisting of layers of the individual polymers but the distribution of the dispersed phase in the continuous phase may not be uniform. Generally, the distribution is determined by the viscosity ratio of the two molten polymers with the higher viscosity component tending to stay close to the center of the molten polymer mass while the lower viscosity component is pushed towards the outside of the extrudate. The shape of the dispersed phase, on the other hand, is influenced by the elasticity ratio between the polymers and the stability of the component distribution by the relative thermodynamics and interfacial parameters of the resins in the blend. Thus, a number of different factors affect the morphology of the final article. Because these factors may not interact to produce a blend of the desired charactertics it will be desirable to evolve some method for improving the production of polymer blends of this kind.